1. Field of the Invention
The invention relates to a method of flaw classification in workpieces, particularly on and in welds of planar or arcuate parts having a known course of thickness, by means of ultrasonics in which a systematical test of the workpiece or its expected flaw areas, respectively, pre-examined manually or mechanizedly if desired, is carried out by using the ultrasound echo image method in separate cross-sectional planes for generating a cross-sectional or B-scan, changing the position of the transducer means on the workpiece and/or the transmitting direction, and calculating the delay times of possible reflections for locating defects.
2. Description of the Prior Art
For non-destructive testing of materials, semi-finished products and finished parts, ultrasonic methods have been in use for quite some time. In connection with the ultrasound echo image method and representation of the result on the screen of a cathode ray tube, there are the possibilities either of generating an A-scan (linear method), where simultaneously with the sound pulse the X-deflection of the cathode ray tube is started and the strength of the received echo effects a Y-deflection so that the abscissa of the blip on the screen contains information above the delay time and thereby the distance between the reflector location and the transmitter location, or of generating a B-scan for a surface representation of a section through the tested body in which reflectors are shown by trace-unblanking. Interpretation of the image in the latter case depends on the experience of the observer.
It has to be an object of any product inspection the production process or of testing parts being in use to provide all information which is either criteria for evaluation of the condition of a product at delivery with respect to a prescribed acceptance standard or is useful for evaluation of the suitability of a part of a machine or of a plant for its further use. While essential advantages of ultrasound technology are the reliable detection and location of internal flaws in materials, and wherein estimation of flaw dimensions can be carried out satisfactorily with certain methodological restrictions, imperfections of ultrasonic testing in the past were due to the qualitative, i.e. flaw-type related interpretation of the natural flaw. Because of an ever growing safety demand and increasing testing activity concerning product tests during production as well as particular tests of highly loaded parts and plants such as high pressure pipe lines, bridge girders and the like, an answer about the flaw type is of essential importance.
It it is rendered possible to classify the flaw type with certainty, on the one hand the affect of a flaw on a construction can be estimated more exactly, but on the other hand, in many cases, particularly in connection with periodical routine inspections of plants in operation, some information can be derived when or under which circumstances a defect might have occured.
With a method for non-destructive testing of a workpiece by means of ultrasonics known from DE-PS No. 32 36 017, it has been tried to render visible with high resolution the shaft of a defect enclosed in the material. This is carried out by utilization of a clocked array of built-up ultrasound transducers, whereby the transmitting direction or angle, respectively, of the array can be varied cyclically, the ultrasound signals being transmitted into a workpiece whose delay time to the reflector on the contour of the defect and the corresponding amplitude are detected and stored. In a second memory, delay time and amplitude of possible locations in the expected flaw area are stored in such a way that only maximum values are written into the memory. The result is then shown on a visual display unit in the form of an intensity distribution (conglomeration of reflection points), wherein one can see where there are essential defects and where there are defects which might be neglected. When applying this known method, information can be obtained as to where in the workpiece there is some discontinuity and which geometric shape the defect indicated by the discontinuity has, based on the measured reflection points. With regard to the type of defect, however no information can be obtained because any connection with the production technology of the workpiece is missing.
Another way of classifying defects occurring in welded plates is disclosed by the article "Flaw Classification in Welded Plates Employing a Multidimensional Feature-based Decision Process" in the periodical Materials Evaluation, Vol. 42, No. 4, pp. 433-438, 443. In accordance with the method disclosed therein, the amplitude of ultrasound echos is recorded while continuously changing the position of the ultrasound transducer or the transmitting direction (angle of impingement), or a magnitude derived from the echo amplitude is recorded in relation to one of the variables or to the delay time between ultrasound transducer and reflector location. Depending on the rate of rise or fall of the echo amplitude at the leading and trailing edges on both sides of a reflector, certain conclusions can be drawn concerning the types of defects, such as cracks, porosities or slags. For rendering possible an objective exploitation of the measuring data when testing a weld, the geometric shape of the weld has to be known since otherwise processing of the measuring data becomes intricate. Therefore, e.g. for testing a double-V weld, the cross-section of the weld is divided into four zones to which the different measuring data are to be allocated. Under such conditions 75% or more of the defects could be identified correctly. With the aid of the known method in expected flaw areas whose extent has to be known at the beginning, the presence of defects can be determined and the type of typical defects can be identified. However, there is not foreseen any type of graphic reproduction for displaying the extent of flaw areas.